Photosensitive resin compositions are generally used for printing plates, and their use is the mainstream in the fields of letterpress printing, lithography, intaglio printing, and flexographic printing.
For such a printing plate, a photosensitive resin layer is brought into close contact with an original pattern film and exposed to light through the original pattern film so as to form portions soluble and insoluble in a solvent, and thus a relief pattern is formed and used as a printing plate.
The printing plate requires a negative or a positive original pattern film, and accordingly the time and cost of its production increase. In addition, since the original pattern film must be developed by chemical treatment and the waste liquid from the development must be treated, the use of the printing plate involves environmental health problems.
A so-called CTP (computer to plate) process in which information processed on a computer is directly output on a printing plate to prepare a relief printing plate without the step of forming an original pattern film has been proposed in association with the advance of computers. In the CTP process, an image mask is formed over a photosensitive resin layer “in situ” with a laser controlled by digital data, and then, the entire surface of the photosensitive resin layer is exposed to active light, typically, ultraviolet light, through the image mask, so that only the regions of photosensitive resin layer not covered with the image mask are selectively cured. The process has some advantages. For example, since this process does not require the above-described step of forming an original pattern film or treatment of waste liquid from development of the original pattern film, it is advantageous in environmental health. In addition, the process can provide a sharp relief.
Specifically, a method of forming an image mask coating on a photosensitive recording component with an ink jet printer or an electrophotographic printer has been proposed (see, for example, German Patent Publication No. 4117127 (Page 1)). Unfortunately, this method cannot provide fine images.
Another method has also been proposed in which a photosensitive flexographic recording material including a photosensitive elastomeric layer, an infrared-sensitive layer opaque to ultraviolet light, and a cover sheet is irradiated with infrared laser light to form an image mask coating (see, for example, U.S. Pat. No. 5,607,814 (columns 17 to 18)). The upper portions of regions irradiated with infrared light of the infrared-sensitive layer adhere to the cover sheet. The regions irradiated with infrared laser light of the infrared-sensitive layer are selectively removed by peeling off the cover sheet. Unfortunately, this method may give damage like scratches to the cover sheet, which can doubles as a protective layer, and thus the damage results in incomplete information transfer disadvantageously. Also, the development by peeling off the infrared-sensitive layer causes the regions unirradiated with infrared laser light to peel easily, and it is therefore unsuitable for the formation of fine image masks.
Even in the field of relief printing, the CTP process has been generally proposed for flexographic printing plate, which are made of an elastomeric binder, such as butadiene rubber or styrene rubber, as a resin, and can use an aqueous ink. On the other hand, in the field of letterpress printing plate, which are made of a soluble resin instead of the elastomeric binder and can use an oil-based ink, there are few methods for the CTP process proposed. This is because the polarities of the infrared-sensitive layer and the photosensitive resin layer, which is made of the soluble resin, are liable to be close, and consequently the infrared-sensitive layer and the photosensitive resin layer are liable to be mixed with time.
In the printing using flexographic plates, the printing between the plate cylinder and the impression cylinder is set weak because the relief to which an image is transferred is soft. The flexographic plate is suitable for printing on corrugated boards having uneven surfaces and flexible packaging films less resistant to high printing pressure. In contrast, in the letterpress printing, the printing pressure between the plate cylinder and the impression cylinder can be set high. This is because the relief is so hard as not to be deformed by high printing pressure and, accordingly, the degradation of printing quality, such as increase in width of letters, is prevented. By increasing printing pressure with use of a letterpress plate, ink can be applied at a large thickness to give a texture of strength to printed material, and metal, to which it is relatively difficult to transfer ink, can be printed on.
For the photosensitive letterpress printing plate using the CTP process, a photosensitive letterpress recording material has been proposed which includes a photosensitive resin layer, an oxygen-transmissive interlayer if necessary, an infrared-sensitive layer opaque to ultraviolet light, and a protective layer (see, for example, U.S. Pat. No. 6,020,108 (columns 11 to 12)). After the protective layer is peeled off, the infrared-sensitive layer is irradiated with infrared laser light to form an image mask. After the entire surface is exposed to ultraviolet light, the image mask and the uncured regions of the photosensitive layer are removed by the same developer. The oxygen-transmissive interlayer prevents mass transfer between the photosensitive resin layer and the infrared-sensitive layer and removal of the photosensitive resin layer by laser engraving. The infrared-sensitive layer is formed by adding an infrared-absorbing and UV-blocking substance, such as carbon black, to a water-soluble or water-dispersible binder. The infrared-sensitive layer, however, does not have crosslinked structures, and is, accordingly, brittle against external flaws. It is therefore necessary to pay attention to handling the material after peeling off the protective layer.
Another method has also been proposed in which the image mask is formed by irradiating with infrared laser light a photosensitive letterpress printing plate precursor including a photosensitive resin layer, a film layer, and an infrared-sensitive layer on a substrate (see, for example, EP Patent Application Publication No. 1152296 (column 26)). In this method, the entire surface of the printing plate precursor is exposed to ultraviolet light through the resulting image mask, and then the image mask is peeled off and removed together with the film layer, followed by water development to obtain a letterpress printing plate. This method does not allow the constituents of the infrared-sensitive layer of the image mask to contaminate the developer and the waste developer is easy to treat, advantageously. However, if the film layer between the photosensitive resin layer and the infrared-sensitive layer has a large thickness, the large thickness easily causes ultraviolet light to curve or disperse. Consequently, if a less directional ultraviolet light source is used, the resulting image may become large undesirably.
In view of the above-described disadvantages, the object of the present invention is to provide a photosensitive resin printing plate precursor capable of forming a protruding relief pattern without using any original pattern film, a method for producing the same, and a method for producing a letterpress printing plate using the same.